In a centrifugal slurry pump, the drive shaft is inserted into the pump to drive the impeller, wich is immersed in the fluid being pumped. A shaft seal is required to dissuade this fluid from leaking out of the pump in any volume.
The challenge is to contain this pressurised and often abrasive liquid using a seal that wil also withstand the friction generated by the rapidly rotating shaft running in the stationary pump housing. The three key types of seals are gland packing, expeller seals and mechanical seals.
The packing needs to be physically in contact with the shaft (or shaft-sleeve) to reduce leakage, this means slightly more power required to drive the shaft or shaft-sleeve.
The packing is worn down over time by friction, which increases leakage until it is adjusted, and eventually replaced.
To work effectively, some of the liquid being pumped needs to leak through the seal for cooling and lubrication. This is a major problem is this liquid has corrosive or other unpleasant qualities.
Constant rubbing means the shaft or shaft-sleeve will wear down over time.
Gland packing is the seal type which requires the most maintenace time, so the cost of maintenance and down time is an important consideration. Also, removing the liquid that has leaded can be costly.
A modification of the gland packing seal is the flushed gland, here clean water is flushed through the bland packing into the pump, which eliminates any leakage and keeps the shaft cool. However, the amount of water required makes this option costlier, plus the liquid being pumped gets diluted which is not always desirable.
Expeller seals
An improvement on gland packing, this is where a rotary expeller (often combined with pump-out vanes on the back of the impeller) reduces the hydraulic pressure at the shaft by pushing the liquid away from it. This can reduce leakage to nearly zero when the pump is operating at optimal conditions. However, when the pump is stationery or running sub-optimally the liquid can still leak. Usually some form of gland packing is used in conjunction with the expeller seal used to minimise such leakage.
Mechanical seals
These are the most effective, and most expensive sealing arrangements for slurry pumps.
A mechanical seal consists of two parts, one section is fixed to the shaft, the other fixed to the stationary pump housing. The interface of these two components is the mechanical seal. One of the sections will be fixed in place, while the other will be pushed up to it using a spring-loaded fixing. This allows a tiny amount of liquid, called the fluid film, to flow into the minute space between these two surfaces, creating a seal, but also providing the needed lubrication and cooling.
This liquid between the two faces of the seal can either come from the fluid being pumped or from an external source. There will be some leakage into the dry area of the pump, but as the gap is so small (as narrow as 1 micron) this will be virtually undetectable.
The advantages of using mechanical seals are:
Cost savings from reduction in wear to pump parts
A reduction in down-time because of repairs and parts are needed less often
A reduction in liquid needed for cooling
The prevention of any discernible leakage of the pumped liquid
The main disadvantage of mechanical seals is their much higher purchase price. However, this is offset by lower running costs and the elimination of any costs associated with leakage. Slurry pumps can be supplied fitted with mechanical seals, or they can easily be retro-fitted.
SlurryPro mechanical seals
These are the most effective, and most expensive sealing arrangements for slurry pumps.
SlurryPro are a global manufacturer of slurry pumps especially for the mining, quarrying and recycling industries.
SlurryPro focus on providing a better slurry pump experience, including superior wear life, reduced lifetime cost and ease of ongoing maintenance. SlurryPro produce centrifugal slurry pumps in a large range of sizes. Pumps can be supplied with gland packing, expeller seals, or the SlurryPro Mechanical Seal.
SlurryPro Mechanical Seals offer:
Complete protection against corrosive leakage
Much increased wear life
Reduced downtime
Reduced clean water consumption
Avoidance of dilution of your process material
This means optimum performance for your plant and a much cleaner working environment.
SlurryPro Mechanical Seals are available as both single and double seals and are designed to meet the arduous requirements of slurry applications. Large radial clearances and advanced multi-spring design give improved seal face loading and longer seal life. They are supplied as a complete cartridge to ensure they are delivered in pristine condition.
Slurrypro Mechanical Seals have been developed and are manufactured by a leading mechanical seal manufacturer. This means you have the confidence you are getting a pump designed for your industry, fitted with a seal from a trusted global mechanical seal manufacturer. A winning combination!